1. Field of the Invention
The present invention relates to a DC-DC converter of a switching regulator system for converting a direct-current voltage, and more particularly to a technique effective for being applied to a DC-DC converter performing a switching drive in a pulse width modulation (PWM) control mode when an input voltage is high and switching the drive system thereof when the input voltage becomes low.
2. Related Art
There is a DC-DC converter of a switching regulator system as a circuit for converting a direct-current input voltage into a direct-current voltage of a piece of different electric potential and outputting the converted direct-current voltage. Among these type of DC-DC converters, there is a DC-DC converter equipped with a driving switching element making an electric current flow through an inductor (coil) by applying a direct-current voltage supplied from a direct-current power source, such as a battery, to the inductor to accumulate energy in the coil, a rectifying element rectifying the electric current of the coil during an energy discharging period, in which the driving switching element is turned off, and a control circuit performing the on-off control of the driving switching element.
Each of these DC-DC converters of the switching regulator systems detects the magnitude of the output voltage of the DC-DC converter with an error amplifier to feed back the detected magnitude of the output voltage to a pulse width modulation (PWM) comparator or a pulse frequency modulation (PFM) comparator, and performs the control of the on-time of the switching element so as to elongate the on-time when the output voltage lowers and so as to shorten the on-time when the output voltage rises.
Furthermore, although the pulse widths of drive pulses are changed according to a load with the period (frequency) of the drive pulses fixed to be constant, the pulses each having a minimum pulse width determined on the basis of the characteristics of the circuit are output even if the load becomes very light by the PWM control. Moreover, if the range of the fluctuation of loads is very large, then the case where the output current of the DC-DC converter is too large even if the DC-DC converter is driven by the pulses each having the minimum pulse width sometimes occurs. Accordingly, as shown in FIG. 4, there is also a DC-DC converter which is equipped with both of a PWM comparator 22 and a PFM comparator 24 and is configured so as to perform the PWM control normally and so as to move the control method thereof to the PFM control for driving the DC-DC converter with fixed pulses each having a constant pulse width and a period changing according to the load of the DC-DC converter when the electric current flowing through the load becomes small, that is, at the time of a light load. There are the inventions described in, for example, Japanese Patent Application Laid-Open Publications No. 2006-149067 and No. 2003-219637 as the inventions pertaining to these DC-DC converters.
In the PFM control mode at the time of a light load of a DC-DC converter performing the drive of the switching element thereof by switching the PWM control and the PFM control as shown in FIG. 4, the output of the PFM comparator 24 changes to a low level when the output of an error amplifier 21 exceeds a reference voltage Vref2 as shown in FIGS. 5A and 5B. Then, as shown in FIG. 5C, the output of an inverter 25 changes to a high level to open an AND gate G1, and pulses from a pulse generating circuit 29 generating the pulses each having a fixed pulse width are supplied to switching elements SW1 and SW2 through a selector 27. The DC-DC converter is configured to thus perform a switching drive.
The DC-DC converter of the PWM/PFM switching system shown in FIG. 4 has an advantage capable of improving the power efficiency thereof at the time of a light load in comparison with a DC-DC converter equipped with only a PWM control function. However, the periods of pulses are longer and the frequency of switching is less by a drive based on PFM control pulses than those by a drive based on PWM control pulses during the period in which an input voltage is comparatively high, but when the potential difference between an input voltage and an aimed output voltage becomes small owing to a consumption of a battery in a system using the battery as the inputting power source thereof, the periods of the pulses become short as shown in FIG. 5E and the frequency of switching increases. Then, there is a disadvantage that the power efficiency lowers because a flow-through current flowing through the switching elements SW1 and SW2 becomes large when the frequency of switching is large.
Moreover, as shown in FIG. 5D, each of the pulses generated by a pulse generating circuit is generally designed so as to have a fixed pulse width and a duty cycle of up to 50%, that is, is designed so that the on-time thereof and the off-time thereof may be the same at a maximum. Consequently, the range of an input voltage is limited by the duty cycle. That is, when the input voltage becomes a certain level or less, the duty cycle of the pulse does not become larger than that at that time. Consequently, it is found that there is a problem of the occurrence of the insufficiency of the electric current flowing through the coil of a DC-DC converter to cause the impossibility of obtaining a desired output voltage.